1. Field of the Invention
This invention relates to a time domain reflectometer (TDR) for measuring characteristics, disconnections and discontinuities etc. of a line.
Particularly this invention is intended for the TDR wherein an output pulse is provided to a transmission line and a reflected signal responding to characteristics of many parts of the line is sampled.
Further, this invention is concerned with the high sensitivity TDR wherein the smaller the reflected signal being detectable is, the more effective the application is.
An example of such application is for an optical time domain reflectometer (OTDR) in which a transmission line under test is an optical fiber.
2. Discussion of Background
In prior art TDR's, particularly the conventional OTDR, a measurement of a long fiber has been required and it has been an important problem to be able to detect the value of the small reflected signal, because a fiber having a very small loss has been produced and a long distance transmission with no repeater is possible due to improvements is light emission elements and photo detection elements.
An example of the prior art is shown in No. 58-120316 of Japanese patent application with the declaration of priority of U.S. Pat. No. 4,438,404, wherein a signal sampling system includes a combined digital and analog acquisition time base for accurately sampling and displaying a short duration electrical event superimposed on a long time duration signal. The signal sampling system is intended for use in stimulus-response situations, and both stimulus and response signals are synchronized with the clock of the digital portion of the time base.
The signal sampling system abovementioned has a precision delay generator including a clock, a digital delay generator and an analog delay generator. The digital delay generator generates a stimulus, such as a light impulse, to be sent down an optical fiber and the analog delay generator generates a sampling signal in precise time relationship with the stimulus.
The longer the fiber under measurement, the larger the attenuation of the fiber and the smaller back scattering optical signal of the fiber. A sampler sampling such a small signal is apt to be influenced by noise. A comparator and a sawtooth generator for generating a linear ramp included in the analog delay generator may be such a noise source. the level of the comparator and the amplitude of the linear ramp are varied according to variations of the sampling point, therefore currents of the comparator and the sawtooth generator are varied and the rise time and amplitude of an output waveform of the comparator is varied slightly but not enough not to be measured and furthermore some influences are given to peripheral circuits of the comparator and the sawtooth generator via power lines and ground.
The analog delay generator, including the comparator, sends the sampling signal to the sampler and the sawtooth generator seriously effects the sampler with an input amplifier amplifying the back scattering signal having a gain of 80 dB to 100 dB or more and the amplified back scattering signal is sent to the sampler. In case sampling occurs long after an optical pulse is sent to the long fiber, undesirable effects may occur because the back scattering signal is very small and therefore a waveform reproduced by sampling does not show the true back scattering signal. In order to eliminate the these effects, filters are used in a power supply and tightly sealed circuits are employed, however they are not totally effective.